Sister Study

Epidemiology Branch

The NIEHS Sister Study ( Spanish ) is prospectively examining environmental and familial risk factors for breast cancer and other diseases in a cohort of 50,000 sisters of women who have had breast cancer. Such sisters have about twice the risk of developing breast cancer as other women. The frequency of relevant genes and shared risk factors will be greater among sisters, increasing the statistical power of the study to detect risks. Sisters are highly motivated, resulting in high response rates and compliance. Thus, studying these sisters enhances the researcher's ability to assess the interplay of genes and environment in breast cancer risk and to identify potentially preventable risk factors. The prospective design allows the assessment of exposures prior to the onset of disease and avoid biases common to retrospective studies. The study—designed to last 10 or more years—also provides a resource to examine a wide range of health outcomes relevant to women, and creates a framework from which to test new hypotheses as they emerge.

Sisterswho were breast cancer-free at enrollment were recruited nationally through health professionals, breast cancer advocates, the Internet, a network of trained recruitment volunteers, and a national advertising campaign. Recruitment strategies were designed to maximize inclusion of minorities and high-risk women. The study is conducted in English and Spanish. Study researchers collected data on potential risk factors and current health status using computer assisted telephone interviews and mail questionnaires. They also collected and banked biological and environmental samples for use in nested studies of women who develop breast cancer (or other diseases) compared with those who do not. Annual tracking and bi- or triennial more-detailed questionnaires update contact information, vital status, and changes in medical history and exposures. Researchers request permission to retrieve medical records and tumor tissue (when relevant) for those who develop breast cancer or other diseases. More than 2,000 women to date have reported a diagnosis of breast cancer since joining the Sister Study Cohort.

Analyses of incident cases will assess the independent and combined effects of environmental exposures and genetic polymorphisms that affect estrogen metabolism, DNA repair, and response to specific environmental exposures. Analyses will focus on known and potential risk factors (e.g., smoking, occupational exposures, alcohol, diet and obesity) and include measurement or analysis of phthalates, phytoestrogens, metals, insulin, growth factors, vitamins and nutrients, and genes in blood and urine. Ancillary studies will investigate the risk for other diseases (e.g., heart disease, osteoporosis, other hormonal cancers, respiratory disease, and autoimmune diseases) and explore genetic and environmental effects on breast cancer prognosis by continuing to follow women in the cohort who develop breast cancer.